Psora 4 is an inhibitor of voltage-gated potassium channels (K1.3; EC = 3 nM). It is selective for K1.3 over K1.1, K1.2, K1.4, K1.7, and K3.1 channels (ECs = 62, 49, 202, 100, and 1,500 nM, respectively) but does inhibit the K1.5 channel (EC = 7.7 nM). Psora 4 selectively binds to the C-type inactivated state of the K1.3 channel and inhibits potassium current in a use-dependent manner. It has immunosuppressive activity, inhibiting proliferation of human and rat myelin-specific effector memory T cells in vitro (TEM cells; ECs = 25 and 60 nM, respectively).